Laser filamentation in air is a subject that is being pursued with great interest all around the world because of potential applications in the areas of directed energy, remote sensing, and artificial lightning. These applications are being pursued by universities, national laboratories, and private companies.
Current methods used to characterize laser filaments are indirect and error prone. B. L. Fontaine et al., “Filamentation of ultrashort pulse laser beams resulting from their propagation over long distances in air,” Phys. Plasmas 6, 1615 (1999), incorporated herein by reference, discusses a method that reflects the filaments from a dielectric mirror inclined at an obtuse angle of incidence and image the surface of the mirror. Using this approach, (i) the mirror is damaged after a small number of exposures and/or (ii) the nonlinear interaction between the filaments and the mirror can strongly perturb the reflected light and therefore corrupt the measurement. Another approach attempts to characterize the plasma created by the filaments. This is done by interferometry, by diffractometry, or by making electrical conductivity measurements between two electrodes. S. Tzortzakis et al., “Time-evolution of the plasma channel at the trail of a self-guided IR femtosecond laser pulse in air,” Opt. Comm. 181, 123-127 (2000) is incorporated herein by reference. H. Ladouceur et al., “Electrical conductivity of a femtosecond laser generated plasma channel in air,” Opt. Comm. 189, 107-111 (2001) is incorporated herein by reference. In practice, such approaches can be error prone because of the uncertainties in the relation between the plasma and the radiation that formed it. Another approach lets the filament drill a hole through a foil and measure the transmitted energy. F. Courvoisier et al., “Ultraintense light filaments transmitted through clouds,” Appl. Phys. Lett. 83, 213-215 (2003). In practice, this approach has several disadvantages including the necessity to guess the relation between the hole size and the filament size, the inability to characterize more than one filament at a time, the inability to characterize the surrounding radiation, and/or the inability to resolve details of the fluence distribution.